I keep hearing folks express an interest in newer, heavier, bullets for the 5.56. It seems to me that the lighter, faster ammo is the way to go. The two schools of thought for ammunition effectiveness have always been "bigger and slower" or "smaller and faster". It seems to me that making a .22 caliber round heavier just makes it smaller and slower. Does anyone besides me feel that the heavier rounds may not be the way to go in .223?  I can see it where penetration is a must (if you must hunt hog or deer with your Ar) but for general use I still feel 55gr is best.  I just want to know why the heavier crowd feels heavy is best.

Coffee

Coffee, depends on the twist rate of your rifle's barrel.  The 55grn bullet is about optimum for 55grn and these days I see the 1:9 twist barrel most often and the optimum there is 62 grn.  The military is able to use the heavier 70+grn bullets as they have a lot of 1:7 twist barrels.

the heavier bullets are giving much better "real world" terminal ballistics.  Twist rate is a major factor however and if you have the 1 in 7, you really do need the 77 gr projectile to make sure they stay down after a hit.  

Heavier bullets retain down range energy better. With the plastic tip technology you still get expansion from a heavy bullet going slow plus penetration.

I like the 75gr Amax